Referring to FIG. 1, computer systems generally comprised of, among other elements, a motherboard (10), a central processing unit (CPU) (12), memory (14), and a plurality of circuit cards (16) for controlling components, performing functions, and the like. Most of these elements are inserted or otherwise electrically connected to the motherboard. Computer system components are generally connected via buses (18) or an electrically conductive path traced along the motherboard. These buses are used for data transfer among the components. Further, power is delivered to the motherboard through a power connection (20). Then, depending on the component, power is supplied indirectly from the motherboard (10) or directly via a power connection on the component. In certain systems, the elements can be removed from or inserted into the computer while the system is running, i.e., the elements can be xe2x80x9chot-swapped.xe2x80x9d
There exist standard specifications that allow the combination of components from different manufacturers. ISA (Industry Standard Architecture) is a bus specification that is based on the specification used in the IBM PC/XT and PC/AT. PCI (Peripheral Component Interconnect) is a local bus specification developed for 32-bit or 64-bit computer system interfacing. Most modem computers have both an ISA bus for slower devices and a PCI bus for devices that need better bus performance. Another specification, VME (VersaModule Eurocard bus), is a 32-bit bus widely used in industrial, commercial, and military applications. VME64 is an expanded version that provides 64-bit data transfer and addressing.
While it is generally cost effective to have most of the circuitry on a single large motherboard for desktop computers, such a configuration has certain drawbacks that are particularly important to industrial applications. Because the motherboard is usually thin and large enough to flex, breakage of small traces and solder joints on fine pitch surface mount devices may occur when plug-in boards are inserted. The occurrence of such breakage requires motherboard replacement, which in turn requires complete disassembly and reassembly of the computer system.
Particularly in industrial applications, such disassembly and reassembly, and the accompanying downtime, may be unacceptable. Also, given the rapid development of motherboard technology, finding an exact replacement for a motherboard can be difficult or impossible. Further, substitution of a non-exact replacement may cause software problems due to BIOS changes, changing device drivers, and different timing. Thus, standard specifications have been developed for systems and boards designed for use in industrial and telecommunications computing applications.
The PCI-ISA passive backplane standard defines backplane and connector standards for plug-in passive backplane CPU boards that bridge to both PCI and ISA buses. The PCI-ISA passive backplane standard moves all of the components normally located on the motherboard to a single plug-in card. The motherboard is replaced with a xe2x80x9cpassive backplanexe2x80x9d that only has connectors soldered to it.
CompactPCI is a specification for PCI-based industrial computers that is electrically a superset of PCI with a different physical form factor. CompactPCI uses the Eurocard form factor popularized by the VME bus.
In the PCI specification, it was possible to select a single value for the pull-up resistor that would satisfy the requirement for both 3.3V and 5V backplanes. Therefore, it was possible to create Universal Signaling Environment capable cards. There is a mechanism defined by the PCI specification where the xe2x80x9csignaling environmentxe2x80x9d of the bus is defined by the value of the pins receiving the input/output (I/O) voltage, i.e., the VIO pins (either 3.3V or 5V). Thus, a universal card uses the I/O voltage VIO to define its own I/O voltage, rather than fixing it at 5V or 3.3V.
The CompactPCI bus architecture supports the 3.3V signaling environment, the 5V signaling environment, and hot swap. These features have the following corresponding requirements. The 3.3 V signaling environment requires 2.7K Ohm (xcfx89) (+/xe2x88x925%) pull-up resistors. The 5V signaling environment requires 1.0Kxcfx89 (+/xe2x88x925%) pull-up resistors. Hot Swap requires that all pins be biased at 1V (+/xe2x88x9220%) using a minimum 10Kxcfx89 pull-up resistor. Further, the Compact PCI specification has the additional requirements of a 10xcfx89 series termination resistor on every signal within 0.6xe2x80x3 of the connector pin, no more than 10 Pico-Farad (pf) capacitive load on any shared bus signal on a non-system slot board, and no more than 20 pf capacitive load on any shared bus signal on a system slot board.
There are two types of xe2x80x9cuniversalxe2x80x9d boards: Universal signaling environment and universal slot location. Universal signaling environment means that a board can operate in either a 3.3V or 5V bus backplane. With the original PCI specification, it was possible to select a value for the bus pull-up resistor that satisfied the specification for both the 3.3V and 5V signaling environments. With the new CompactPCI Specification, it is no longer possible to select a single resistor. Therefore, in order to be a universal signaling environment capable CompactPCI board, a board must provide both 2.7Kxcfx89 (+/xe2x88x925%) and 1.0Kxcfx89 (+/xe2x88x925%) pull-up resistors and provide a way to enable them correctly depending on the signaling environment.
Universal slot location describes a board that can function in either the system slot or non-system slot of a CompactPCI backplane. A system slot board is required to provide the common bus resources for the CompactPCI backplane, namely: bus pull-ups, ebus clock, and the bus arbiter. A system slot board is allowed additional capacitive load per signal pin because of these additional features. In order to be CompactPCI Hot Swap Specification compliant, every signal pin must be biased to (1V +/xe2x88x9220%) through a minimum 10Kxcfx89 resistor prior to insertion into a live or xe2x80x9chotxe2x80x9d backplane.
Those skilled in the art will appreciate that other requirements exist in the full CompactPCI, Hot Swap, and Passive Backplane PCI-ISA specifications which are available from PCI Industrial Computer Manufacturers Group of Wakefield, Ma. and are hereby incorporated in their entirety by reference.
In one aspect, a device for automatically providing variable resistance comprises a comparator for comparing a reference voltage to an operating voltage; a first switch operatively coupled to the comparator; a first resistor operatively coupled with the first switch in a series connection between a pull-up voltage and a signal line; a second switch operatively coupled to the comparator; and a second resistor operatively coupled in a series connection with the second switch between the pull-up voltage and the signal line. The first switch selectively electrically enables the connection between the pull-up voltage and the signal line through the first resistor based on the comparison between the reference voltage and the operating voltage and the second switch selectively electrically enables the connection between the pull-up voltage and the signal line through the second resistor based on the comparison between the reference voltage and operating voltage.
In one aspect, a method of automatically providing variable resistance comprises comparing a control voltage and a reference voltage; selectively pulling-up a signal line to a pull-up voltage through a first resistor and a first switch operatively connected in series if the comparison has a first outcome; and selectively pulling-up the signal line to the pull-up voltage through a second resistor and a second switch operatively connected in series if the comparison has a second outcome.
In one aspect, an apparatus for automatically providing variable resistance comprises means for comparing a control voltage and a reference voltage; means for selectively pulling-up a signal line to a pull-up voltage through a first resistor and a first switch operatively connected in series if the comparison has a first outcome; and means for selectively pulling-up the signal line to the pull-up voltage through a second resistor and a second switch operatively connected in series if the comparison has a second outcome.
In one aspect, a system for automatically varying resistance comprises a voltage supply for supplying a reference voltage and an operating voltage; a signal line requiring a pull-up resistance of a differing value depending on the operating voltage; a comparator for comparing the reference voltage and the operating voltage; a first switch operatively coupled to the comparator; a first resistor operatively coupled with the first switch in a series connection between a pull-up voltage and a signal line; a second switch operatively coupled to the comparator; and a second resistor operatively coupled in a series connection with the second switch between the pull-up voltage and the signal line. The first switch selectively electrically enables the connection between the pull-up voltage and the signal line through the first resistor based on the comparison between the reference voltage and the operating voltage and the second switch selectively electrically enables the connection between the pull-up voltage and the signal line through the second resistor based on the comparison between the reference voltage and the operating voltage.
In one aspect, an apparatus for providing variable resistance, comprises a voltage supply for supplying a reference voltage and an operating voltage; a signal line requiring a pull-up resistance of a differing value depending on the operating voltage; a comparator for comparing the reference voltage and the operating voltage; a first switch operatively coupled to the comparator; a first resistor operatively coupled with the first switch in a series connection between a pull-up voltage and a signal line; a second switch operatively coupled to the comparator; and a second resistor operatively coupled in a series connection with the second switch between the pull-up voltage and the signal line. The first switch selectively electrically enables the connection between the pull-up voltage and the signal line through the first resistor based on the comparison between the reference voltage and the operating voltage and the second switch selectively electrically enables the connection between the pull-up voltage and the signal line through the second resistor based on the comparison between the reference voltage and the operating voltage. The signal line requires a bias voltage at an insertion time. Also included are a bias control signal indicating the insertion time; and a third resistor operatively coupled in a series connection with a third switch between a bias voltage and the signal line. The third switch is operatively coupled to the bias control signal and the third switch selectively electrically enables the connection between the bias voltage and the signal line through the third resistor based on the bias control signal. The signal line requires a connection to an auxiliary output terminal at a dual-load time. Further included are a dual-load control signal indicating the dual-load time; and a fourth switch operatively between the auxiliary output terminal and the signal line. The fourth switch is operatively coupled to the dual-load control signal and the fourth switch selectively connects the signal line and the auxiliary output terminal based on the dual-load control signal. Other aspects and advantages of the invention will be apparent from the following description and the appended claims.